Bone formation is a complex and highly organized process. How the bone forming osteoblasts are regulated during bone formation is only partially understood. Impaired osteoblast activity has been associated with many bone diseases, including osteoporosis, and it is likely that a better understanding of osteoblast regulation will contribute to early detection of disease and the identification of useful drug targets. Our long range goal is to gain insights into the regulation of osteoblasts under physiological conditions in living bone. Therefore, we plan to demonstrate that animal models permit non-invasive assessment of osteoblast regulation in vivo. We propose to develop innovative mouse models that harbor reporter gene marked osteoblasts and lack specific target genes. Differential analysis of the reporter will allow determination of osteoblast activity by a novel non-invasive imaging technique in living mice as well as by modular ex-vivo multi-modality analysis. Thus, we should be able to examine the dynamic cellular events that cause changes in bone formation. We will test the hypothesis that mechanical loading affects osteoblasts in vivo and that the early growth response gene 1 (EGR-1) is essential for this response. The aim is to determine the consequences of controlled axial compression of the mouse tibia on osteoblasts in the presence or absence of EGR-1 by means of both classical and reporter dependent ex-vivo analysis. To overcome the limitations of analyzing post mortem, we then plan to take advantage of the reporter gene by utilizing positron emission tomography to perform repeatedly non-invasive real-time visualization of osteoblast dynamics in the bone of living mice. The proposed studies will serve as a model for future studies that aim to dissect osteoblast regulation by targeted in vivo analysis of specific signaling pathways. Moreover, successful use of non-invasive molecular imaging may lay the foundation to further develop this method to study bone biology in vivo. [unreadable] [unreadable] [unreadable] [unreadable]